Buck-Boost converters are used in applications where the desired output voltage (e.g. 3.3V) can be lower or higher than input (generator) voltage (e.g. Vg=2.5V−5.5V). FIG. 1A prior art gives an example of a typical buck-boost switch configuration. This configuration is called noninverting or positive buck-boost converter as the output voltage has the same sign as the input voltage.
The basic operating principle of a typical buck-boost converter is shown in FIG. 1A prior art. When both the switches are in phase-1 the inductor is connected to supplies and is charged with current and when both the switches are in phase-2 the inductor current charges the output capacitor. The output voltage vs. input (supply) voltage (Vg) as a function of duty cycle in this mode of operation for boost operation is given by:
                              Vout          Vg                =                  D                      1            -            D                                              (        1        )            
For buck operation the formula is:
      Vout    Vg    =  D
Thus the converter is capable of achieving output voltages in buck-boost operation lower or higher than the input voltage.
FIG. 1B prior art gives the topology of an output stage of a synchronous buck-boost controller with integrated switches.
Referring back to basic operation of FIG. 1A prior art, in phase-1 M1 and M3 will be on, M2 and M4 will be off; and in phase-2 M2 and M4 will be on and M1 and M3 will be off.
Comparing the basic Buck-Boost operation with a typical Buck or Boost converter, we can list the following disadvantages:                4 switches change state at each cycle, thus switching loss is 2 times of a typical Buck or Boost converter        The average inductance current is significantly higher than the load current, given as:IL=ILOAD/(1−D) (e.g. when D=0.5, Vg=Vout, IL=2×ILOAD), which leads to                    increase in inductor current            increase in resistive losses (e.g. for D=0.5 example, losses due to RESR,L will be 4 times of a Buck                        At phase-1 only the capacitor is sourcing the load, thus a low ESR cap is needed.        Higher current ripple on inductor.        
There is a growing demand for wearable electronic devices. However for wearable applications line transient is an important specification as the battery is very small. Voltage Mode control needs complex feedforward techniques to improve line transient response.
Hysteretic control is a preferable control method for good line transient response. Hysteretic control has other advantages such as design simplicity, ultra-low power operation and unconditional stability. Wearable applications load current is relatively low, therefore more suitable for hysteretic control.
It is desirable to have a Hysteretic Control Buck-Boost converter which employs “separated Buck and Boost pulses” thus leading to                higher efficiency,        lower inductance current ripple,        simple design,        good line transient performance, and        lower quiescent power.        
It is a challenge to designers to fully exploit the opportunities outlined above.